1. Field of the Invention
This invention relates to a plunger and barrel assembly for a fluid system which effectively minimizes leakage through a clearance between the plunger and the barrel assembly.
2. Description of the Related Art
Engine designers are continually seeking improvements in engine design which improve engine efficiency. One manner of improving engine efficiency is to improve the operational efficiency of the fuel system. Specifically, any leakage of high pressure fuel within the fuel system represents wasted energy that can reduce engine efficiency. Loss of high pressure fuel has recently become an even greater problem as injection pressure levels are increased in an effort to improve fuel economy and reduce emissions as required by recent and upcoming legislation.
Undesirable leakage of fuel often occurs in a component of the fuel system having a member, such as a valve element or a fuel plunger, reciprocally mounted in a bore formed in a body and sized to form a close sliding fit with the inside surface of the body to create a partial fluid seal between the adjacent surfaces. As the fuel pressure increases, a pressure gradient is developed along the length of the seal, i.e., clearance, between the member and opposing wall forming the bore. The extent of the leakage flow through the clearance depends primarily on the magnitude of the pressure gradient, the engagement length, the size of the operating clearance and the fluid viscosity. The size of the operating clearance is affected by the amount of fuel pressure induced dilation or deformation of the body forming the bore. One manner of reducing the leakage is to design the components to achieve a smaller clearance between the plunger and barrel. However, the practice of requiring closer tolerances increases manufacturing costs. Another method of reducing leakage is to design the body to resist pressure induced dilations by increasing the size and/or strength of the body or housing forming the bore. However, this method undesirably increases the size and weight of the components and, thus, the fuel system.
Many fuel systems used in contemporary engines include a reciprocally mounted fuel pressurization plunger incorporated into, for example, a unit fuel injector, such as disclosed in U.S. Pat. No. 5,072,709, or a fuel pump assembly, such as disclosed in U.S. Pat. No. 4,530,335. Each plunger is typically either mechanically or hydraulically operated to pressurize fuel in a pressure chamber for injection into the engine cylinder. For example, U.S. Pat. Nos. 5,096,121 and 5,441,027 disclose hydraulically actuated intensification plunger assemblies. However, these references do not suggest reducing the leakage between the plunger and adjacent bore wall and, therefore, are subject to the disadvantages discussed hereinabove.
U.S. Pat. No. 4,991,495 to Loegel, Sr. et al. discloses a pumping mechanism including a plunger mounted in a bore and a plurality of inserts positioned in series along the plunger for sealing the space between the plunger and its housing. The inserts include thrust and sealing rings which deform and expand radially in response to axial fluid-induced forces imparted by adjacent inserts.
U.S. Pat. No. 5,038,826 to Kabai et al. discloses a three-way valve including a piston slidably positioned in a valve body. High pressure fuel is delivered to the valve via aligned ports formed in the valve body and the piston. An integral portion of the piston or the valve body is acted upon by supply fuel pressure to reduce the clearance between the piston and a valve body thereby reducing the leakage between the components. Although deformation of the integral portion tends to close the clearance gap to reduce leakage, the resulting close tolerances may result in increased wear, or possibly scuffing, of the valve body or piston resulting, over time, in excessive clearances. For the Kabai et al. design, excessive wear would eventually require replacement of the entire piston and/or valve body, unnecessarily increasing costs. Also, the integral portion disadvantageously provides reduction in the pressure gradient over only a limited, localized portion of the seal length and thus fails to minimize leakage in an optimum manner. In addition, the integral portion is formed by machining internal passages into the valve body or piston undesirably increasing manufacturing time and costs.
U.S. Pat. No. 3,954,048 to Houser discloses a high pressure, self-sealing and self-lubricating, reciprocating pump having a pair of uniformly thin wall, radially resilient, cylinders extending in parallel into adjacent cavities of a pump housing. Pistons is slidable in the cylinders. The outer surfaces of the cylinders form annular spaces in the cavities which communicate with pressure chambers in a manifold operatively connected to the pump housing. Pressure changes due to compression and suction in the pump causes the thin wall cylinder to collapse and expand about their respective pistons forming thereby a high pressure seal during compression, and a self-lubricating cylinder during suction.
Finally, U.S. Pat. No. 5,899,136 to Tarr et al. which is also assigned to the assignees of the present invention, and the contents of which are incorporated herein by reference, discloses a plunger reciprocally mounted in a cavity formed in a barrel, and a leakage flow reduction device positioned in the cavity for reducing fluid leakage flow around the plunger, thus increasing system efficiency. The leakage flow reduction device includes a sealing sleeve removably mounted in the cavity between the plunger and the barrel. The sealing sleeve includes a bore for slidably receiving the plunger to form an annular clearance gap between the plunger and the bore. The sealing sleeve is designed to resiliently flex in response to fluid pressure forces to reduce the annular clearance gap so as to minimize fluid leakage through the annular clearance gap. The sealing sleeve is formed as a separate piece from the barrel to permit simple, low cost replacement.
However, both Houser and Tarr references disclose a sealing sleeve that deflects inwardly under pressure to reduce the annular clearance between plunger and the barrel, to thereby minimize fluid leakage through the annular clearance gap during the compression stroke of the plunger. While use of such inwardly deflecting sealing sleeves provide various benefits, there still exists a need for a further improved fluid control device which effectively and optimally minimizes fluid leakage through the clearance between a plunger and a barrel, while minimizing the costs and size of the device.